This invention relates generally to gas turbine engines and more particularly to shroud for a turbine of such engines.
Gas turbine engines include one or more turbines which extract energy from a gas flow to produce mechanical work. It is common in the prior art for a structural exhaust frame or turbine rear frame (“TRF”) to be disposed behind a low pressure turbine (“LPT”). Because of the configuration typically used, the transition between the structural case of the LPT and the structural case of the TRF must provide an inward-sloping diameter.
This inward slope from the LPT case to the TRF case creates a radial pocket with a forward axial boundary created by the shroud surrounding the last stage of LPT blades and an aft boundary created by the inward-sloping contour of the TRF case. In the event of a turbine failure in which debris is ejected aftward, this pocket can trap a quantity of the debris and prevent it from being ejected aft out the tailpipe of the engine. Entrapment of debris from a turbine failure in this fashion is highly undesirable, since damage could conceivably sever the engine carcass load path to the aft mount. This, in turn, could lead to engine component liberation and aircraft damage.
The presence of the inward-sloping frame case directly aft of the LPT exit blade tip seal is also subject to impingement of high-temperature leakage air which bypasses the blade across the tip seal teeth Since this cycle air has had no work extracted by the last LP stage, it will be significantly hotter than the remaining cycle air to which the TRF is exposed. It will also have higher impingement velocity and therefore heat transfer coefficient relative to the remainder of the LPT discharge flow. Locally, this can create adverse thermal gradients and thermal fatigue issues in the TRF case, in addition to overtemperature concerns. The radial transition also requires a length of conical structure aft of the shroud axial position, which adds length and weight to the engine.
Accordingly, there is a need for a turbine shroud arrangement which protects the TRF case from debris and high temperatures.